This invention relates to rigid disks used for memory storage in computers. In particular, this invention relates to disk substrates upon which layers of magnetizable materials are applied.
Aluminum alloys have long been used as substrate materials for rigid computer memory disks due to aluminum's light weight, high strength, wide availability, low cost and good surface finishing characterisitics. Advances in computer technology, however, have imposed more stringent requirements on these disks, particularly smaller thicknesses and greater areal density of information. To meet these needs, the smoothness of the substrate has become more critical.
Smoothness is particularly problematical in view of the need for high yield strengths. The disks must retain a high yield strength despite being subjected to annealing for thermal flattening and stress relief prior to application of the magnetizable layer. Existing aluminum disk technology achieves the high yield strength by incorporating magnesium in the alloy to a level of about 4%. Such high magnesium contents tend to combine with the silicon impurity normally present in aluminum alloys to form magnesium silicide constituents which are detrimental to the surface characteristics of the substrate.
The solution offered by the present invention relates to the use of zinc as an alloying element. Related literature on aluminum-zinc-magnesium alloys includes a disclosure by Westerman et al., U.S. Pat. No. 3,542,606 (Nov. 24, 1970), in connection with articles of heavy section prepared by processes which do not involve thermal flattening or annealing. Similar alloys are disclosed by Murmann, U.S. Pat. No. 684,707 (Oct. 15, 1901), where the purpose was to achieve alloy products free from pores; Fuller et al., U.S. Pat. No. 1,578,979 (Mar.30, 1926), where the purpose was to achieve cast alloys of superior machining properites; Brown et al., U.S. Pat. No. 3,674,448 (July 4, 1972), in connection with a composite anode material; McGinnis et al., U.S. Pat. No. 3,825,993 (July 30, 1974), in connection with clad product which retains its strength at high temperatures and even benefits from the exposure; and Peters et al., U.S. Pat. No. 3,466,156 (Sept. 9, 1969), in connection with multilayered magnetic tapes and discs.